A substrate holder of the type mentioned at the beginning is described in U.S. Pat. No. 5,457,398. It has first and second elements that are electrically insulated from one another, the first element of the substrate holder having an upper surface for the horizontal accommodation of a test circuit for testing by an electric probe. The substrate holder is surrounded by a sheath. This arrangement is provided for a triaxial structure. For this purpose, the second element has upper and lower parts, each of which consists of conductive material. In addition, there is applied a first non-zero potential difference between the first element of the clamping device and the outer sheath and a second non-zero potential difference, which is substantially equal to the first potential difference, between the second element of the clamping device and the outer sheath. Hence, the first element carries a force potential, the second element carries a guard potential and the sheath carries a shield potential.
The growing number of applications in the field of measurement of low currents in the femtoampere range (10−15 A) and in the attoampere range (10−18 A) makes the development of increasingly sensitive measuring equipment necessary. The increasing number of sources of interference (e.g., increasingly stronger wireless message transmission) makes increasingly better shielding or other measuring and compensating methods also necessary. Added to this are additional parameters (e.g., temperature of the substrate) that must be kept spatially and temporally constant with great accuracy during the measuring process. Well thought-out arrangements with careful selection of material are required to obtain spatial constancy of the corresponding specified parameters. Precise and reliable control technology provides appropriate temporal parameter constancy.
The measurement of small currents with rapid signal rise times requires a measuring arrangement with small parasitic capacities in the signal path and high insulating resistances over the entire temperature range. The observance of parameter temperature necessitates good thermal contact of the substrate. Along with all the rest of the measuring arrangement (e.g. measuring chamber, probe heads and probe needles), the substrate-holding arrangement, or chuck, is of great importance.
U.S. Pat. No. 5,610,529 discloses a structure of a substrate holder that has an electrically conductive holding plate. This holding plate adjoins an insulating plate. Below the insulating plate, there is provided a so-called thermochuck, which provides for the addition or removal of energy to or from the substrate, depending on whether it is to be tested at high or low temperatures.
Here, addition or removal of thermal energy may be effected conventionally or alternatively by gases or liquids controlled externally and conveyed through the thermochuck, as described in U.S. Pat. Nos. 4,426,619 and 4,734,872.
In this arrangement, the holding plate carries a force potential and the thermochuck a guard potential.
It has been found that such an arrangement produces non-homogeneities in the conduction of heat to the substrate. As a result, a frictional electric effect, which results in interference with the measurement, cannot be avoided here.